MM-45956: Optimize FileInfo stats query (#22603)

* MM-45956: Optimize FileInfo stats query

We Denormalize Post.ChannelId on FileInfo.ChannelId

```release-note
The file info stats query is now optimized by denormalizing the channelID column into the table itself. This will speed up the query to get the file count for a channel on clicking the RHS.

Migration times:
On a MySQL 8.0.31 DB with
1405 rows in FileInfo and 11M posts, it took around 0.3s

On a Postgres 12.14 DB with
1731 rows in FileInfo and 11M posts, it took around 0.27s
```

https://mattermost.atlassian.net/browse/MM-45956

server/channels/app/file_test.go

@@ -368,6 +368,7 @@ func TestSearchFilesInTeamForUser(t *testing.T) {
 			fileInfo, err := th.App.Srv().Store().FileInfo().Save(&model.FileInfo{
 				CreatorId: th.BasicUser.Id,
 				PostId:    th.BasicPost.Id,
+				ChannelId: th.BasicPost.ChannelId,
 				Name:      searchTerm,
 				Path:      searchTerm,
 				Extension: "jpg",